This invention relates to a process and apparatus for perforating a film of thermoplastic material and containers made therefrom. More particularly, this invention relates to a process and apparatus for microperforating a zippered film of thermoplastic material useful for manufacturing reclosable zippered produce bags.
Thermoplastic films, for example polyethylene films, have been used in many different applications including the packaging industry for making wraps for wrapping food articles or bags for storing food articles.
Non-microperforated zippered plastic containers or bags which feature reclosable fasteners are also well known and widely used by consumers for storage of foods. Typically, opposing rib and groove fastener elements (also called male and female profiles) on the container are pressed together or pulled apart to seal or open the container. An example of these reclosable containers or bags are ZIPLOC.TM. (trademark of DowBrands Inc.) brand bags.
Typically, the reclosable bag is made by first forming a film web stock, fusing profiles to the web stock integrally or separately, folding the web stock and then heat sealing the web stock to form the bag. Generally, the reclosable plastic bag is made from a clear thermoplastic film web or sheet typically of a polyethylene resin material. Thus, the resulting bag product is generally a clear plastic bag. Generally, the industry utilizes an extrusion die in which a closure profile of a thermoplastic resin is extruded and subsequently joined to the thermoplastic film web or sheet. Alternatively, the closure profile elements and sheet may be extruded as an integral unit from a single die whereupon the closure profile elements and sheet fuse to form an integral plastic container stock material.
The process of manufacturing the bag first involves forming a film web stock. Generally, a film web stock comprising an integrally fused closure member and film web is formed by first supplying a source of extrusion resin material for the closure member and the film web member to extruders. The extruders feed the resin into a die member to coextrude the closure device and film web integrally as the closure device and film web exit the die onto the surface of a chill roll. Air jets and an air knife are used to assure good contact of the film and closure device fastener elements with the chill roll. The film is pressed uniformly on the surface of the chill roll by the air jets and air knife. The air jets above the air knife pin the edges of the drawn film extrusion to the chill roll. Below the air knife preferably is located water jets which are aligned with each of the closure device fastener members to cool and shape the fastener members.
The closure integrally fused with the film web, herein web stock, is formed and chilled on the chill roll and from the chill roll, the web stock may go through a series of orientation, tension, nip and/or idler rolls to direct the web stock to pass through an apparatus for folding the web stock and joining the closure member profiles together to form a bag stock which is ready for severing and sealing into individual bag products.
The process of manufacturing thermoplastic bags or containers typically involves supplying a continuous web of the thermoplastic material which has been folded upon itself to form two plies to a means for severing and sealing the two plies into individual bag products. The bag stock is sent to a bag sealing machine or means for making the bag product such as described in U.S Pat. No. 5,203,556. In forming individual bags, portions of the thermoplastic material are severed from the web. These severed areas become the side seams for the bags and are typically sealed at the same time as they are severed by the use of a heated wire element.
In some applications it is desirable for a thermoplastic film to be perforated to be used as wrap film, for example, in wrapping applications where air permeability through the wrap film is desirable. Therefore, such perforated wrap film can be used for wrapping produce such as vegetables and fruits wherein the free flow of air to the produce is desired to minimize early spoilage of the produce.
Typically, the thermoplastic film is perforated using a microperforator machine. Microperforator machines useful for perforating thermoplastic film are well known in the art such as described in U.S. Pat. No. 4,667,552.
The microperforator machines described in the prior art, for example, U.S. Pat. No. 4,667,552, are used to make microholes on flat non-zippered film after the film is manufactured. However, the known microperforators are not designed for processing film web stock with any protrusions, protuberances, ribs or profiles on the surface of the film web. More particularly, the known microperforators are not designed to accept a web stock having zippered profiles of the kind used to make zippered bags or containers. A problem with microperforating zippered film is the difficulty in obtaining uniform microholes across the surface of the zippered film. It is difficult to control the uniformity of the hole size especially when strict specifications are required for microperforating a film, for example, when a specific density and distance of the microholes is required for use in manufacturing a produce bag.
There is still a need in the industry to produce microperforated thermoplastic film containing zippered profiles with better control of microhole size on the web and to manufacture reclosable zippered bags from the microperforated zippered film material.
Accordingly, it is desired to provide a process and apparatus for microperforating a zippered web stock useful for making zippered reclosable produce bags. It is further desired to provide a continuous in-line perforating operation for zippered film and a process for producing a microperforated zippered bag.